Analog semiconductor lasers, such as InP Distributed Feedback (DFB) lasers, are finding increasing use in cable TV transmission systems. In one design, the laser is mounted on the surface of a silicon substrate, and the light from the laser is focused onto an optical fiber by means of a spherical lens which is mounted in a cavity in the silicon substrate. (See, e.g., U.S. patent application of Anigbo, Ser. No. 60/00916, filed Dec. 22, 1995, which is incorporated by reference herein.)
One of the problems associated with fabricating such devices is to keep distortion ripple at a minimum. The "distortion ripple" is defined as the variation of the values of the composite second order distortions and the composite triple beat distortions. Generally, it is desired to keep the distortion ripple at or better than 4 dB over a laser temperature variation of 2 deg C., which is the typical temperature variation of the laser in the package.
Recently, it has been suggested that distortion ripple can be reduced by making at least a portion of the spherical lens optically asymmetric to prevent multiple reflections of scattered light within the lens. (See U.S. patent application of Bylsma et al, Ser. No. 09/069109, filed Apr. 29, 1998, which is incorporated by reference herein.)